Sensor elements to be used in pressure sensors can be designed to detect strain or capacitance, i.e. be the strain type or the capacitive type. They can be built from various ceramic materials. Ceramic materials based on aluminium oxide are often used in such sensor elements, but glass ceramic materials can also be used. A ceramic capacitive sensor element for sensing pressures is usually constructed of two main parts. These parts comprise a stabile base plate and a thinner circular plate, also called a diaphragm, a part of which is movable with a pressure difference and which mounted to one of the large surface of the base plate and joined thereto, by for example, glass joints at the circular edge of the thin plate and the base plate. The diaphragm has the same diameter as the base plate and has a thickness which is adapted to the magnitude of the load, i.e. the pressure difference to which the diaphragm is intended to be subjected.
The change of the position of the central portion of the diaphragm is detected as a change of a capacitance between two oppositely located electrodes made of, e.g., gold, the electrodes being layers which are coated by means of thin film methods on facing surfaces of the base plate and the diaphragm respectively. Such a sensor element can be used for different types of pressure measurements, where the desired variable is a measurement pressure acting on the free surface of the diaphragm, i.e., on the surface which does not face the base plate. The measurement is made in relation to some form of a reference pressure acting on the inner surface of the diaphragm which faces the base plate and is opposite the free surface. Pressure sensors can be classified based on the way in which the reference pressure is formed. Thus the pressure sensor is:
a “gauge sensor” if the reference pressure=the atmospheric pressure
an “absolute sensor” if the reference pressure=a technical zero pressure
a “differential sensor” if the reference pressure=a second measurement pressure
The diaphragm of such a sensor element is the part which mainly determines the performance of the sensor element. The diaphragm should be as thin as possible in order to provide a high sensitivity. However, a too thin diaphragm can easily break if exposed to too high pressures. Apparently a sensor element intended for measuring extremely small pressures should comprise such a very thin diaphragm, but such a diaphragm cannot without taking some precautions be subjected to the atmospheric pressure. However, thin plates, which have been produced by mechanical working such as polishing, always have mechanical stresses and are not completely flat in an unloaded state, at least not for varying ambient temperatures, and are thus not suitable to be used in high reliability pressure sensors which are intended for small pressures.